A progressive cavity (“PC”) pump is located in a well and is used to pump subterranean fluids to the surface. Well over 12,000 oil wells in Canada use PC pumps. Conventionally the PC pump is driven from a speed-reducing drivehead or top drive located at the surface. A long assembly or string of rods transmit the slow rotation from the top drive down the well to the PC pump. The top end of the rod string is fitted with a polish rod which is driven through a rod clamp which is driven by the top drive.
The conventional rod string rotational speed of 30–700 rpm is adjusted according to the downhole pumping conditions including the oil characteristics. Lower rotational speeds are not currently practiced due to the large gear reduction required and lack of flexibility to attain greater speeds as pumping conditions improve. Under certain conditions, such as in the case of very viscous oils or low formation influx rates, it desirable to have even lower rotational speeds. These cases are not adequately served with the prior art top drives.
When driven, the rod string winds up in torsion like a rubber band. In a 3,000 foot long rod string, the rods could be wound over 100 revolutions. When the top drive ceases to drive the rod string and coupled PC pump (such as during a power failure), the rods try to unwind, spinning the conventional drive and motor backwards, called backspin. If unrestrained, backspin speed of the rods can reach upwards of 5,000 rpm and, in combination with gear reduction to the top drive (now acting as a speed increaser), the resulting speed at the top drive and motor can dangerously destructive—with most injuries being caused by sheave self-destruction.
Apparatus is known to control backspin. Some top drives have braking systems which are activated upon backspin, either when the drive shuts down or the rod string physically begins to spins backwards. Substantially all modern top drive are fitted with some sort of device which deals with backspin.
Older top drives utilize centripetally-actuated mechanical drum-breaking systems. Alternately a hydraulic pump can cause brake pads to engage a disc-braking rotor on the polish rod. Braking shoes, drums, shoes, pads and discs are subject to wear and the possibility of localized hot spots which can be an ignition hazard.
Some top drives are hydraulic motor driven. Upon backspin the drive naturally reverts to a pump. Hydraulic fluid is directed through a restriction orifice or check valve for releasing pressure. Similarly, in motor-driven gear boxes, a clutch activates on backspin to engage a separate hydraulic pump. At the high torque of backspin conditions, the hydraulic fluid can become very hot, or if the fluid is lost, backspin control is also lost.
In another instance, it is known to provide an AC drive motor with an inline shaft. A Variable Frequency Drive or VFD manipulates line AC/DC and then DC back to a braking. Slip associated with AC induction motor unfortunately limits the motor's ability to reach the low rotational speeds desired in some circumstances, such as with high viscosity oils. Low rotational speeds can result in motor overheating.
Further, to start an AC induction motor under load can result in in-rush amperage as much as 5 or 6 times the normal full load operating current. For instance, for a typical 50 Hp NEMA B motor having a full load current of about 200 A, the in-rush could be 1000–1200 amperes. While this is only for a short period of time and is an accepted disadvantage of AC motors, there is a further economic impact. Often, the peak current use of an end user is identified by the utility provider as the basis for setting the per unit charge for all power consumption. Accordingly, even though steady-state use may be only 200 amperes, the price paid for the electricity could be based on a higher, albeit momentary, electrical current use.
Thus, there is a need for a more versatile top drive which is capable of a wide range of operating speeds, particularly low rotational speeds and is does not suffer the enumerated backspin disadvantages of the gear-reduced apparatus.